The present invention generally relates to a field apparatus for transmitting a signal through a pair of transmission lines. In particular, the present invention relates to a field apparatus that allows easy transition from an analog signal transmission system to a field bus transmission system.
Normally, the so-called field apparatus means equipment for detecting physical quantities such as a pressure, a temperature and a fluid delivery rate at a plant, then converting the magnitude of the quantity into an electrical signal and finally transmitting the electrical signal to a higher-rank apparatus through a transmission line as well as for reversely receiving a control signal from the higher-rank apparatus and controlling a valve or another device at the plant in accordance with the control signal.
In the case of an electrical signal transmitted as an analog signal, the transmission of the electrical signal conforms to certain standards. That is to say, the transmitted analog electrical signal which is referred to hereafter simply as an analog signal has to be in the range 4 to 20 mA DC. In addition, the analog signal is generally communicated between the field apparatus and a higher-rank apparatus in one direction.
By virtue of the progress in semiconductor integrated circuit technology made in recent years, however, a field apparatus employing an embedded microprocessor has been developed and put to practical use. As a result, it is now possible to communicate digital signals in both directions in addition to the communication of an analog signal in one direction through the transmission line described above and to set the range of the field apparatus as well as carry out self diagnoses on the field apparatus by using commands transmitted from a remote location.
Examples of an intelligent field apparatus of this type are disclosed for example in Japanese Patent Laid-open No. Sho 58-48198 and Japanese Patent Laid-open No. Sho 59-201535.
A concrete example is explained by referring to FIG. 7. FIG. 7 is a diagram showing an analog signal transmission system employing analog signal oriented field apparatuses and an intelligent field apparatus.
The operations of the analog signal oriented field apparatuses 1ad and 1bd and the intelligent field apparatus 1id employed in the transmission system shown in FIG. 7 are driven by electric power supplied by an external power supply 4d. Detected physical quantities are each converted thereby into an electrical current which is then transmitted as an analog signal through a transmission line 5.
In a higher-rank receiving apparatus 3d, the current flows through a resistor connected in series to the transmission line 5. The analog signal is detected by the higher-rank receiving apparatus as a difference in potential between the ends of the resistor which is not shown in the figure. The analog signal is received as a quantity transmitted by the field apparatus 1ad, 1bd or 1id.
Installed between the intelligent field apparatus 1id, the higher-rank communication apparatus 3d and the external power supply 4d, a communicator 2 is used for facilitating communication between the intelligent field apparatus 1id and the higher-rank communication apparatus 3d in both directions.
A system of transmitting a digital signal by superposition of the digital signal on an analog signal without affecting the analog signal, a system of signal transmission by switching from analog to digital and vice versa and a system of transmission by transmitting only a digital signal are commonly known as systems for transmitting signals through the transmission line 5.
In addition, a field bus transmission system has been proposed recently as a system for exchanging digital signals only in both directions to and from a plurality of field apparatuses connected to a common transmission line.
A representative example of the configuration of the field bus system is explained by referring to FIG. 8. FIG. 8 is a diagram showing a typical configuration of the field bus transmission system.
As shown in the figure, a plurality of field apparatuses are connected to a higher-rank communication apparatus by a common transmission line to form a tree like shape.
Driven by electric power supplied by an external power supply 4 through a transmission line 5, field apparatuses 1add, 1bdd and 1cddoperate, exchanging digital signals in turn with a higher-rank communication apparatus 3 also through the transmission line 5 in both directions in processing to transmit detected physical quantities and to receive control values to and from the higher-rank communication apparatus 3.
Provided between the field apparatuses 1add, 1bdd and 1cdd, the higher-rank communication apparatus 3 and the external power supply 4, a communicator 2 is used for facilitating the exchanging of the digital signals between the field apparatuses ladd, lbdd and lcdd and the higher-rank communication apparatus 3 in both directions. A terminator 7 provided at each end of the transmission line 5 comprises a resistor and a capacitor connected to each other in series.
In order to migrate from the transmission system shown in FIG. 7 to the field bus transmission system shown in FIG. 8, it is necessary to replace the field and higher-rank communication apparatuses by those designed for the latter system. However, the migration from one system to another can be accomplished with ease, because the transmission line 5 can be used as it is. In addition, since the number of field apparatuses connected to the transmission line 5 in the field bus transmission system shown in FIG. 8 can be increased, the system can be expanded easily.
When migrating from an analog signal transmission system to a field bus transmission system, however, it is necessary to change everything except the transmission line 5 at once. As a result, an effort to use of a field bus transmission system is not commensurate with as many merits as expected by the user when seen from the cost performance point of view.
In order to solve this problem, a virtual field apparatus was developed as disclosed in Japanese Patent Laid-open No. Hei 4-195500. The virtual field apparatus is designed so that the existing analog signal oriented field apparatus can be used as it is even if a migration is made from an analog signal transmission system to a field bus transmission system.
That is to say, in a migration from an analog signal transmission system to a field bus transmission system, the virtual field apparatus is provided between the transmission line 5 and a plurality of existing analog signal oriented field apparatuses. Analog signals output by the analog signal oriented field apparatuses are converted into digital signals by the virtual field apparatus and then undergo predetermined signal processing before being transmitted through the transmission line 5.
By adding one virtual field apparatus having such functions, a plurality of existing analog signal oriented field apparatuses can be used in a field bus transmission system.
In the event of a failure occurring in a virtual field apparatus disclosed in Japanese Patent Laid-open No. Hei 4-195500 due to some reason, however, it is quite within the bounds of probability that a plurality of analog signal oriented apparatuses connected to the virtual field apparatus can not all be used.
In addition, a period during which effects of the introduction of a virtual field apparatus can be obtained is limited to a period to migrate from an analog signal transmission system to a field bus transmission system. In addition, functions that a field bus oriented field apparatus is capable of implementing such as autonomous control of distribution of tasks among field apparatuses, preventive maintenance and retrieval of apparatus information like self diagnosis results can not be executed by the virtual field apparatus.